Heatsinks, mylar washers, thermal paste, safety...

Discussion in 'General Electronics Chat' started by guitarguy12387, Sep 3, 2010.

  1. guitarguy12387

    Thread Starter Active Member

    Apr 10, 2008
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    Hey guys,

    I just had a few questions about this stuff, mostly with regards to safety.

    Okay, presently when i need to put a heatsink on a TO-220 vreg or whatever, i have simply used some mylar washers for electrical isolation. I was always a bit weary about this practice from a safety standpoint, however. For example, say i have a high voltage regulator and i am using a heatsink with mylar washers. What is to prevent the heatsink mounting screw from touching the inside of the mounting holes of both the heatsink and the reg??? Then, you have high voltage sitting on the heatsink! Not good. What if i am then mounting the heatsink to the case? REALLY not good. But this seems to be standard practice! What am i missing!?

    Also, how does thermal compund factor in? I've never had the need to use any up till now. According to wiki, it can be electrically conductive...

    Help me not shock myself haha!
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    There are mounting kits available like the one at - http://www.mouser.com/Electromechanical/Electronic-Hardware/Heatsinks/_/N-6g7nr/?No=200 The image is the second from the bottom of the list. That washer insulates the 4-40 mounting screw from the tab.

    The advisability of using an insulator depends on the regulator. If the tab is a ground, it makes no difference. Negative regulators will usually need an insulator, although if they come in the isowatt package, they are already insulated.

    With a power transistor, there may be a higher voltage. The film is usually enough, but it's best to check the insulating ability is you are up to hundreds of volts.
     
  3. guitarguy12387

    Thread Starter Active Member

    Apr 10, 2008
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    Ohh okay. I suppose using things like this or using plastic screws or something would work.

    See, that's what i figured too... but i mean... what if i use the washers, and check the isolation with a DMM and some mechanical vibration causes the heatsink/reg tab to rest on the screw?! Seems like that is WAY to risky to do, especially in the hundreds of volts and especially if your "heatsink" is the chassis of the device.
     
  4. BMorse

    Senior Member

    Sep 26, 2009
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    most heatsink insulator inserts, would have a lip that goes into the hole of the TO-220 style tabs, this insulates the screw from touching the tab...

    what I have done in the past when I didn't have any of these types of washers on hand, was to cut a piece of plastic/nylon tubing, I would then insert a piece of this inside the TO-220 tab hole and then run the screw through that to insulate it from the tab along with a couple of flat nylon washers.

    [​IMG]

    B. Morse
     
  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Guitarguy a regular flat washer should NOT be used. You should use a "shoulder" washer like what B. Morse just posted. The shoulder goes into the hole and prevents the side of the screw from touching the sides of the holes. The shoulder washer should be as close as possible to filling in the whole hole depth without being higher than the height of the mounting tab. Plastic "creeps" if the shoulder is too deep you are guaranteed to have your screw loosen with time/heat leading to a failed product.

    Also depending on what you mean by high voltage a shoulder washer might not suffice. If this is a commercial product UL (or other standards agency) will investigate your shoulder washer/screw/regulator for creepage/clearances which because a shoulder washer doesn't full encase the hole the gap on the heatsink side might allow reduced creepage/clearance distances which can lead to arcing and failures.
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    and plastic screws should never be used as again plastic "creeps" and will loosen rather quickly leading to poor connection to the heat sink and it all goes down hill from there.
     
  7. bertus

    Administrator

    Apr 5, 2008
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  8. guitarguy12387

    Thread Starter Active Member

    Apr 10, 2008
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    @bertus, Thanks for the link! I'll have a read through that.

    @mcgyvr, Noted! Thanks for the advice. Curious why they even make the TO-220 shaped flat mylar washers?! Good advice on the plastic parts as well. For the record, i'm talking in the neighborhood of 200-300v for tube amps and such.

    @BMorse, THATs more like it! That would make me feel alot better.


    So how does thermal paste work into the equation? Seems like thats just asking for a short btw sink and tab. (pre-emptively, i did read the bit on thermal paste in the above link... but it still seems like a huge gamble).
     
  9. BMorse

    Senior Member

    Sep 26, 2009
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    there are some thin sheets of Mica / silicone pads that you can place in between the heatsink and the tab, the thermal compound is applied to both sides of this pad to allow for thermal transfer from tab to heatsink....

    [​IMG]

    B. Morse
     
  10. beenthere

    Retired Moderator

    Apr 20, 2004
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    Just for the record, that mounting kit in post #2 contains the insulating pad as above, plus the shoulder washer, a 4-40 screw, flat washer, lock washer, and 4-40 nut.
     
  11. DonQ

    Active Member

    May 6, 2009
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    Yeah, one of the previous post sounded like you wanted to leave out the mylar washer. You still need to isolate the TO-220 from the heatsink, as well as insulating the screw from the heat-sink.

    There are also washers that have a similar electrical insulating effect, but are better heat conductors than a dry mylar washer (without heatsink grease). They are usually made from silicon and are malleable so they don't need grease. I think in the end they are slightly less heat conductive than a proper mylar-grease setup (check the numbers if it's critical), but it's often worth it to get rid of the mess that grease can cause. (Reminds me or working with permatex. Yuk!)

    The heatsink grease is only meant to fill the tiny voids between the mylar and the metal on either side. It squeezes out of any pressured areas, only leaving tiny portions where air would otherwise be, then these tiny areas conduct heat better than if air was there. It would also work metal-to-metal on applications where the heatsink can be directly connected to the case of the device. But it is never meant to provide electrical insulation.

    If you are really worried about heat conduction, there are mechanical things you can do to minimize voids between the two surfaces that can even give you more advantage than using the grease. Making sure the surfaces are flat, de-burred, ultra-smooth, etc.
     
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